You would have to create the image using an external program. When it's done, import the PNG into a new archive in SLADE 3, and create a new lump called TEXTURES. You may have to click "Edit as Text" and set the language to ZDoom Textures (assuming that's the port you're using)

The images should be very large, I think Reinchard666 is making his hi-res texture pack 8x larger than the originals.

Therefore, a 128x128 texture would have to be 1024x1024. Once this is done use something like this in TEXTURES

code:texture BACONTEX1, 1024, 1024
{
XScale 8
YScale 8
}

That's a very basic version of code. You can find more about TEXTURES here.

EDIT: The XScale and YScale literally just define a scale factor for how big the texture should appear in-game. They get smaller, so a scale of 8 would make the texture 8 times smaller in game. I guess the bigger the original texture, the better it will look, but I would've said 8x is more than enough. Remember XScale and YScale don't have to be the same value.

DOUBLE EDIT: If this is for GZDoom, don't convert the PNG to a palette. It's not necessary and will look shit.

Has anyone successfully made high resolution sky textures yet? I'm sure anyone who has made a sizeable outside area has noted that the larger the courtyard gets, the greater the pixels in the sky become...

I'm basically playing with some moon base geometry, and was thinking to cheese in a high resolution starfield, if at all possible.

Anyone who drops in a successful hint on how to do this will get a thank you in the credits for "Doom 1.5 Hell on The Moon", when it's done.... ;P